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Home , 8th parallel north, Aries (constellation), Capricornus, Celestial equator, Constellation, Declination

2. Descriptive (“Astronomy Without a Telescope”)

http://apod.nasa.gov/apod/astropix.html • How do we locate in the heavens?

• What stars are visible from a given location?

• Where is the in the at any given time?

• Where are you on the ? An “” is two stars that appear To be close in the sky but actually aren’t In 1930 the International Astronomical Union (IAU) ruled the heavens off into 88 legal, precise . (52 N, 36 S)

Every , , etc., is a member of one of these constellations.

Many stars are named according to their and relative brightness (Bayer 1603).

Sirius α − Centauri, α-Canis less http://calgary.rasc.ca/constellation.htm - list than -53o not Majoris, α-Orionis visible from SC http://www.google.com/sky/ https://en.wikipedia.org/wiki/List_of_Messier_objects (1758 – 1782) Biggest constellation – – the female water snake 1303 square degrees, but and almost as big.

Hydrus – the male water snake is much smaller – 2243 square degrees

Smallest is – the Southern Cross – 68 square degrees Brief History

Some of the current constellations can be traced back to the inhabitants of the Euphrates valley, from whom they were handed down through the Greeks and Arabs. Few pictorial records of the ancient constellation figures have survived, but in the AD 150, catalogued the positions of 1,022 of the brightest stars both in terms of celestial and , and of their places in 48 constellations.

The Ptolemaic constellations left a blank area centered not on the present south pole but on a point which, because of , would have been the south pole c. 2800 BC, a fact that is consistent with the belief that the constellation system had its origin about 5,000 years ago. E.g.,

Betelgeuse and are  M42 = Orion M43 = DeMairan’s nebula and -Orionis Winter Triangle – brightest star in the sky – star of about twice the 8.6 ly mass of the sun. Blue. Very luminous, very hot. A star (like the sun) but of Type A1

Procyon – 8th brighest star. About 1.4 solar masses. Another 11.5 ly main sequence star. Hotter and more luminous than the sun but not as luminous as Sirius. Type F5. May be close to finishing burning as its is a bit high for its mass.

Betelgeuse – 9th brightest star. 2nd brightest in Orion. 643 ly Betelgeuse is a red supergiant. It is not fusing hydrogen in its center. It has left the main sequence. May vary in brightness over periods of years by as much as a factor of two. About 18 solar masses and around 10 million years old. Winter Hexagon : M-42 1600 light years away in the sword of Orion, easily visible to the . 85’ x 60’ across and part of a larger cloud spanning 20 degrees*. Diameter ~30 ly, Mass ~ 200,000 solar masses.

• Your fist at arm’s length is about 10 degrees

Star Nursery Betelgeuse - red supergiant, about 20 solar masses. May have shrunk 15% in radius since 1993. This probably does not indicate evolution at its center. 570 ly away. . 1000 times as luminous as the sun Rigel - brightest star in Orion by (a bit more than -Orionis = Betelgeuse – a variable) 7th brightest star in the sky. 770 ly. Most luminous star in our region of galaxy. A blue , 17 solar masses. Brightness varies by 3 to 30% Triple . A is bright. B is a binary. Trapezium - an of young stars which illuminate the Orion nebula. The 5 brightest are all over 15 solar masses. Three were discovered by Galileo in 1617.

optical IR

*Bayer (1603) designated the brightness of 1564 stars in his Finding the north star

x 6

Your latitude is the angle above the northern horizon where you see . Polaris does not move. Motions of stars in the sky

North South Polaris is 6 times the distance between the pointers away – i.e., ~30o. Can tell time this way, but a) 24 hr clock b) A siderial day is a bit shorter than a solar day

24 hr 365.25 The apparent location of the sun in the sky as the earth goes round it defines a great circle in the heavens called the “”.

The projection of the earth’s in the sky gives another called the “”. Because the Earth’s rotational axis is not perpendicular to the plane containing the earth’s around the sun, the planes containing the two circles are not the same but are inclined to each other by 23.5o.

The path of the sun in the sky

XI/30 – XII/17 Think of the earth as being at the center of this imaginary

23.5o

The ecliptic is fixed in the sky because the earth’s orbit doesn’t change. The celestial equator moves over 1000’s of years due to precession Where the sun is Astronomically speaking. The traditional signs of the are all 30 degrees in length and the sun spends roughly equal times in each. They are not equivalent to where the sun is. E.g., the zodiacal sign (1/21– 2/19) roughly corresponds to Capricon below.

Pisces The March 12 to April 18 The Ram April 19 to May 13 The Bull May 14 to June 19 The Twins June 20 to July 20 The Crab July 21 to August 9 The Lion August 10 to September 15 Virgo The Maiden September 16 to October 30 The Balance October 31 to November 22 The November 23 to November 29 ** -holder November 30 to December 17 The Archer December 18 to January 18 The January 19 to February 15 Aquarius The Water-bearer February 16 to March 11

Because the and sun are all approximately in the same plane, the planets are also found in the constellations of the zodiac. uses the tropical zodiac which is affixed to the vernal . The siderial zodiac is fixed to the stellar background when the system started. uses the siderial zodiac https://en.wikipedia.org/wiki/Zodiac - Constellations How about the apparent motion of the stars in the sky?

If you stood at the earth’s , your would be the projection of the earth’s rotational axis into the sky.

Your horizon would be the celestial equator.

The stars would go round Polaris in a counter- clockwise direction The celestial equator is the projection of the Earth’s equator into the heavens. 8 hr time lapse photo The Daily Motion

• As the Earth rotates, the sky appears to us to rotate in the opposite direction. • The sky appears to rotate around the N (or S) celestial poles. • If you are standing at the poles, nothing rises or sets. • If you are standing at the equator, everything rises & sets 90o to the horizon. copied from Nick Strobel’s At the equator, stars would “Astronomy notes”. See his website. all rise perpendicular to the horizon and set perpendicular to the horizon. Every day Panoramic view of the African from equatorial Kenya. The three hour long exposure was made on a clear, dark, mid November evening facing due west and covers just over 180 degrees along the horizon. So, the South is at the center of the concentric arcs on the left and the North Celestial Pole is at the far right. The stars setting along the Celestial Equator leave the straight trails near the middle of the picture.

Leroy Zimmerman, Astronomy picture of the day November 15, 2002 Latitude of Seattle At a lower latitude than the north pole = 47.6 degrees

90 + 47.6 = 137.6o

180 − 90 − 47.6 = 42.4 47.6

42.4o

Stars within a certain angle of the north pole would go in circles around the pole and never set. Others have more complicated paths. Some near the south pole remain invisible. Only stars on the celestial equator would rise due east and set due west. Stellar Coordinates and Declination

• Celestial Equator Projection of the Earth’s equator into the sky

• Declination − 90o ≤δ ≤ + 90o The angle to a star or other object in degrees, minutes, and seconds measured north or south of the Celestial Equator N

 CE

S What can be seen from a given location (e.g. Santa Cruz)? L = 37 Celestial equator (90o away from poles) Polaris  = 0 Sometimes 37o visible 90-L -53o North 37o South 37o

E.g., at Santa Cruz where latitude = 37o N

o Stars more than 53 above the South pole Celestial Equator will always be visible. Stars more than 53o below the CE will never be visible. The poles and the celestial equator actually latitude of SC is 36.792o` remain fixed in the sky as the earth rotates Stars will be “circumpolar”, i.e., never set if their declination is L = latitude o δ ≥ 90 − L L > 0

in the and δ ≤ − 90o − L L < 0 in the southern hemisphere. Note that L is negative in the . At the south pole L = -90. At the north pole L = +90. A star will rise above the horizon sometime in a 24 hour period if

Northern o e.g, north pole  > 0 hemisphere δ > L − 90 south pole  < 0 Southern o equator 90 >  > -90 hemisphere δ < L + 90

Where  is the declination of the star and L is your latitude. 90 ≥ δ ≥ − 90 90 ≥ L ≥ − 90

L = -90 is the south pole. L = 90 is the north pole, 37 is Santa Cruz. L - 90

90 - L Brightest Stars Day Solar The Altitude of the Sun Declination

~March 21 0 ~June 21 23.5o ~September 0 21 - ~December -23.5o 21

dates are approximate and vary from year to year

To the “tropics” – below 23.5o 23.5o What can be seen from a given location (e.g. Santa Cruz)? L = 37 Celestial equator (90o away from poles) Polaris  = 0 Sometimes 37o visible 90-L -53o North 37o South 37o

E.g., at Santa Cruz where latitude = 37o N

o Stars more than 53 above the South pole Celestial Equator will always be visible. Stars more than 53o below the CE will never be visible. The poles and the celestial equator actually latitude of SC is 36.792o` remain fixed in the sky as the earth rotates copied from Nick Strobel’s “Astronomy notes”. See his website. How do we assign a location to a star in the sky?

We could say so many degrees above the horizon and so many degrees east or west from some point, like the southern direction, but a little thought shows that location would vary with location and time on the Earth. How we define our location on the Earth...

Prime Santa Cruz latitude = 37o N longitude = 122o W An important location in the sky, to astronomers, is the “Vernal Equinox”, where the center of the sun crosses the CE.

23.5o

Vernal equinox The vernal equinox is in Pices. The autumnal equinox is in Virgo. Astrologers would say Libra (offset by the precession of the ). UT = Greenwich England Stellar Coordinates

• Celestial Equator Projection of the Earth’s equator into the sky

• Declination The angle to a star or other object in degrees, minutes, and seconds measured north or south of the Celestial Equator − 90o ≤δ ≤ + 90o • Right Ascension The angle measured eastwards from the Vernal equinox along the Celestial equator to the of the star. Measured in units of time (1 hour = 15 degrees; 1 minute of time = 15’ of angle) 0h ≤ RA ≤ 24h

Measuring angles in units of time? A convention used in astronomy because of historical reasons.

Declination is measured in degrees (and minutes and seconds), but Right Ascension (RA) is measured in hours, minutes, and seconds…. sorry…

1 hr of RA = 15 degrees of ordinary angular measure (360/24)

1 min of RA = 15/60 = ¼ = 15 arc min of angular measure i.e., 15 degrees is one hour and one minute is 1/60 of that 1 sec of RA = 15/3600 = 1/240 degree = 15 arc sec

nb. 0 longitude on Earth is defined by Greenwich England. 0 right ascension in astronomy is defined by the vernal equinox in Pices copied from Nick Strobel’s “Astronomy notes”. See his website. Brightest Stars

http://www.google.com/sky/

Actual Coordinates of Polaris:

Declination = 89o 15’ 51” RA = 2h 31m 48.7s Examples

Sirius:  = -16o 43’; RA = 6 hr 45.2 min

-Centauri:  = -60o 50’; RA = 14 hr 39.6 min http://www.google.com/sky/

How many degrees is 14 hr 39.6 min? 1 hr = 15 degrees 1 min = 15’

14 hr*(15 0/hr) + 39.6 min (15’/min) = 210 0 594’

but 594’/60’ per degree = 9o with 54’ left over

so 14 hr 39.6 min or RA is 219o 54’ East of the Vernal Equinox

This is also 360o – 219o 54’ = 140o 06’ West of the Vernal Equinox

NAVIGATION Your Celestial Meridian is the imaginary line through your zenith and north (or south pole) from horizon to horizon.

Your siderial time is equal to the right ascension of stars on your CM. At midnight siderial, the vernal equinox is on your CM

Your longitude is the difference between your local siderial time and the siderial time in Greenwich. (subtract Greenwich siderial time from your local siderial time).

To navigate in the old days your prime need was a good clock (if the sky was clear) and knowledge of the stars. The Longitude Prize was a reward offered by the British government for a simple and practical method for the precise determination of a ships longitude. The prize was established through an (the Longitude Act) in 1714 and was administered by the Crude example

The longitude of New York is 74 degrees W. We are 4 time zones west of New York Each time zone is 15 degrees (360/24) So we are roughly at longitude

74 + 15*4 = 134

Actually we are at 122 W, but this shows the idea. E.g. RA of Betelgeuse is 05 h 55m 10.3053 s Suppose Betelgeuse crosses your CM when the siderial time in Greenwich is midnight (0h 0 m)

Your longitude is 5 h 55m … or 5.920 h or 88.79 degrees

You are 88.79 degrees east of Greenwich. Positive numbers are east (definitely not Santa Cruz; possibly )

(Time is later as you go east, e.g., NY vs Santa Cruz)

Aside, the vernal equinox is on your CM at “midnight” siderial time (not necessarily at night). Siderial time is defined as the “” of the vernal equinox. Precession of the Equinoxes

Due to the interaction of an earth that is not perfectly spherical with the gravitational pull of the sun and 1.38 degrees per century

As a result of this precession the projection of the earth’s equator into the sky - the celestial equator - also moves and this causes an adjustment of the equinoxes. This in turn changes the coordinate system in which a star’s location is measured. The vernal equinox drifts westward along the ecliptic about an arc minute per year (actually 50.35 arc seconds).

So when a star’s coordinates are given (RA and ), a date must also be given. Current tables use 2000 as a reference point.

Corrections to where to point a telescope are discussed at e.g., http://star-www.st-and.ac.uk/~fv/webnotes/chapt16.htm